Over the last years, a cellular communications network structure known as the Internet of Things has emerged. Generally, this network structure comprises a huge number of small autonomous devices, which typically, more or less infrequently (e.g. once per week to once per minute) transmit and receive only small amounts of data, or are polled for data. These devices are sometimes referred to as Machine Type Communication (MTC) devices, Machine-to-Machine (M2M) devices or just Machine Devices (MDs), and are assumed not to be associated with humans, but are rather sensors or actuators of different kinds, which typically communicate with application servers (which configure and receive data from the devices) within or outside the cellular network.
With the nature of MTC devices and their assumed typical uses follow that these devices generally will have to be energy efficient, since external power supplies not necessarily are available and since it is neither practically nor economically feasible to frequently replace or recharge their batteries. In some scenarios the MTC devices may not even be battery powered, but may instead rely on energy harvesting, i.e. gathering energy from the environment, opportunistically utilizing (the often very limited) energy that may be tapped from sun light, temperature gradients, vibrations, etc.
So far, the MTC related work in 3rd Generation Partnership Project (3GPP) has focused on MTC devices directly connected to the cellular network via the radio interface of the cellular network. However, a scenario which is likely to be more prevalent is that most MTC devices connect to the cellular network via a gateway. In such scenarios the gateway acts like a User Equipment (UE) towards the cellular network while maintaining a local network, typically based on a short range radio technology towards the MTC devices. Such a local network, which in a sense extends the reach of the cellular network (to other radio technologies but not necessarily in terms of radio coverage) has been coined capillary network and the gateway connecting the capillary network to the cellular network is thus referred to as a capillary network gateway (CGW). Hence, the capillary network comprises one or more CGWs and a plurality of MTC devices, which connect to a Radio Access Network (RAN) of an available cellular communications network via the one or more CGWs.
Radio technologies that are expected to be common in capillary networks include e.g. IEEE 802.15.4 (e.g. with IPv6 over Low power Wireless Personal Area Networks (6LoWPAN) or ZigBee as higher layers), Bluetooth Low Energy or low energy versions of the IEEE 802.11 family (i.e. Wi-Fi). A capillary network may be single hop (i.e. all MTC devices have a direct link to the CGW), e.g. a Wi-Fi network with the CGW as the access point, or multi-hop (i.e. some MTC devices may have to communicate via one or more other MTC devices to reach the CGW), e.g. an IEEE 802.15.4+ZigBee network with the CGW being a Personal Area Network (PAN) controller. In multi-hop cases, the Routing Protocol for Low-Power and Lossy Networks (RPL) may be used. In principle, RPL may be used also in single hop networks, but there is less need for a routing protocol in such networks.
The field of capillary networks is still not abundantly explored and many issues are still to be resolved. For instance, a problem to be solved is to how handle authentication, identification and/or secure communication of the MTC devices towards the cellular network.